Chaff discrimination system

ABSTRACT

1. A microwave fuze system for a missile including a firing circuit having means for discriminating between a target and a countermeasure reflector, comprising in combination, a frequency modulated transmitter, an antenna coupled to said transmitter and radiating a signal polarized in a predetermined direction, a pair of receiving antennas including one antenna polarized in the same direction as said transmitting antenna and one antenna polarized in a direction normal to said transmitting antenna, a first switch means coupled to said receiving antennas and adapted to alternately connected said receiving antennas to a mixing means whereby the received signals are mixed with a portion of the transmitted signal, means for amplifying a band of said mixed transmitted and received signals, a pair of envelope detector means connected to said amplifying means by a second switch means, a difference amplifier adapted to receive the output from said pair of envelope detector for coupling a firing signal to said firing circuit when the energy level of the output of a certain one of said detectors exceeds the energy level of the other detector output, and a periodic wave control means actuating said first and second switch means whereby said pair of receiving antennas and said pair of envelope detectors are synchronously connected to said mixing means and said amplifying means respectively.

United States Patent n 1 Adrian 1 CHAFF DISCRIMINATION SYSTEM [75]Inventor: Donald J. Adrian, Arlington, Calif. [73] Assignee: The UnitedStates of America as represented by the Secretary of the Navy,Washington, DC. [22] Filed: Mar. 6, 1959 [21] App], No: 797,816

[52] US. Cl. H 343/7 PF; 343/5 R; 343/18 E [51] Int. Cl. I, GOlS 9/02;HO4K 3/00 [58] Field of Search 343/18, 18 D, 5, 1711, 343/5 R, 17.1 R,18 R, 18 E, 7 PF [56] References Cited UNITED STATES PATENTS 2,472,2126/1949 Hudspcth 343/18 D 2,508,571 5/1950 Hudspeth 343/1741 FOREIGNPATENTS OR APPLICATIONS 750,600 6/1956 United Kingdom 343/5 805,97712/1958 United Kingdom I 1 343/5 Primary E.rarninerMaynard R. W'ilburAssistant ExaminerGi E, Montone Artorney, Agent, or Firm Richard SSciascia; Joseph M. St, Amand; T. M. Phillips EXEMPLARY CLAIM 1. Amicrowave fuze system for a missile including a firing circuit havingmeans for discriminating between a target and a countermeasurereflector, comprising in combination, a frequency modulated transmitter,an antenna coupled to said transmitter and radiating a signal polarizedin a predetermined direction, a pair of receiving antennas including oneantenna polarized in the same direction as said transmitting antenna andone antenna polarized in a direction normal to said transmittingantenna, a first switch means coupled to said receiving antennas andadapted to altemately connected said receiving antennas to a mixingmeans whereby the received signals are mixed with a portion of thetransmitted signal, means for amplifying a band of said mixedtransmitted and received signals, a pair of envelope detector meansconnected to said amplifying means by a second switch means, adifference amplifier adapted to receive the output from said pair ofenvelope detector for coupling a firing signal to said firing circuitwhen the energy level of the output of a certain one of said detectorsexceeds the energy level of the other detector output, and a periodicwave control means actuating said first and second switch means wherebysaid pair of receiving antennas and said pair of envelope detectors aresynchronously con nected to said mixing means and said amplifying meansrespectively.

1 Claim, 3 Drawing Figures 44 HORIZONTAL L ENVELOPE 34 DETECTOR w 46 4339 3s 37 H I f FIRING DIFFERENTIAL FERRITE cIRcuIr l AMPLIFIER /45SW'TCH M M'XER SWITCH 35 v VERTICAL ENVELOPE DETECTOR MODULATOR H U.S.Patent Nov. 4, 1975 Sheet 2 of3 3,918,059

N b\| mm mm xv @3380: R V mm Iota w M wcmmmz m XE EEESZ 5:5 W km mm mm9v 2 Q vv mohouhmo nv EEUEHEE Iv h3 0 @ZEE mokuukmo US. Patent Nov. 4,1975 Sheet 3 of3 3,918,059

NM NM mmxzz mokomkmo @ZEE mohomhmo CHAFF DISCRIMINATION SYSTEM Theinvention described herein may be manufactured and used by or for theGoernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

This invention relates to a chaff discrimination system for a missilefuze and more particularly to a means for discriminating between atarget and chaff by utilizing the difference in polarization propertiesof the echoes in a radio fuze system.

One of the most common countermeasures used by target aircraft toprematurely fire the radio fuze of a missile is to dispense a pluralityof small metallic particles, commonly known as chaff, which will reflectthe transmitted carrier wave in the same manner as the target. Formissiles that are air-launched and normally are steered to the rear ofthe target aircraft in a tail chase, the chaff is dispensed rearwardlyfrom the target aircraft, while the head-on attacks by surface-launchedmissiles the chaff cloud is provided by a forward fired chaff-dispensingrocket launched from the target.

The present invention involves the difference in polarization of themicrowave signal reflected from a target and chaff. When chaff isilluminated with radiation by a transmitted microwave of any one of thecommon radio fuze systems employed in missiles and the radiation has forexample, vertical polarization only, there is both a horizontal andvertical polarization in the return echo signal from the chaff. On theother hand, if simple metal surfaces common to the extremities of mostmodern airplanes or missiles where fuzes usually function such as aplane, a cylinder or a sphere are illuminated with vertically polarizedradiation, the reflected echo signal consists only of radiation havingvertical polarization.

The present invention consists essentially of a bias channel identicalwith the fuze receiver channel of a typical microwave radio fuze systemsuch as the system disclosed by Whitely and Adrian in their applicationSer. No. 566,318 filed Feb. I7, 1956 and entitled Random FMAutocorrelation Fuze System or the Noise Modulated Fuze System of AdrianSer. No. 761,447 filed Sept. 16, 1958. The transmitted carrier wave ofsuch systems will have a single polarization pattern of, say horizontalwhereby the reflected target echo will have only a horizontal componentand is thus received, amplified and detected by the fuze receiverchannel. It the detected signal is of sufficient amplitude, the fuzefiring circuit will be actuated. The bias channel of the instantinvention has a receiving antenna of, say vertical polarization wherebythe reflected signal from chaff which has both a horizontal and verticalpolarization component will be received, mixed with the transmittedsignal, amplified, detected and fed to the firing circuit to bias thefiring signal from the fuze receiver channel when the verticalcomponents from the chaff is of sufficient magnitude to balance out thehorizontal component detected by the fuze receiver channel.

It is an object of the invention therefore, to provide a positive meansfor a fuze system to discriminate between 2 target aircraft and chaff.

Another object of the invention is to provide an improvedcounter-countermeasure device for a radio fuze system of a missile.

A still further object of the invention to provide a means forpreventing the firing of a missile fuze system when chaff is detected.

A further object of the invention is to provide a means for detectingthe polarization components of a reflected radio signal to distinguish atarget signal from a countermeasure signal.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

FIG. I is a block diagram illustrating one preferred embodiment of theinvention;

FIG. 2 is a block diagram illustrating a second preferred embodiment ofthe invention;

FIG. 3 is a block diagram illustrating a third preferred embodiment ofthe invention.

Referring to the drawings and with particular reference to FIG. 1 thisembodiment comprises a transmitter 10 which has its carrier modulated byany common modulator l1 employed in microwave fuze systems, such asfrequency modulation by white noise or a periodic wave. The signal isradiated in the direction of the target by a horizontally polarizedantenna l2.' A portion of the transmitted RF signal is also applied toeach of the balanced mixers l3 and 14 where it is combined in the mixer13 with the return signal from the target aircraft which has beenmodified by the Doppler effect due to the relative movement between themissile and aircraft and is received through the horizontally polarizedantenna 15 and applied to the mixer 13.

The output from the mixer 13 is passed through a band pass amplifier 16which is usually designed to pass a band of expected Doppler frequencieswhich in turn are passed through a detector 17. The output of thedetector 17 is the envelope of the band pass amplifier output and is feddirectly to the differential amplifier 18 to fire the fuze ashereinafter described. The system just described is a typical radio fuzesystem of a missile and is assumed to be mounted in a moving missile(not shown) which is guided in a path to approach a target with the RFenergy from the fuze being transmitted toward the target with a returnecho signal being received therefrom. The transmitted carrier from thetransmitter 10 need not be modulated but could be non-modulated carriersignal from the horizontally polarized antenna 12.

The bias channel, which can be added to many of the known radio fuzesystems and is shown in the dashed line portion 25 of FIG. 1, comprisesthe mixer 14 which combines a portion of the transmitted signal with areturn echo signal from chaff when encountered that has a verticalpolarization component and is received by the vertically polarizedantenna 20. The bias channel is the same as the receiver channel 13-17of the fuze system described above in that the output from the mixer 14is passed through a band pass amplifier 21 which is set to pass a bandof expected Doppler frequencies. The output from the amplifier 21 isdetected by the envelope detector 22 and the envelope of the detectedsignals is applied to the differential amplifier 18 to cancel the signalfrom 17 and thus prevent a positive signal from being fed to the firingcircuit 26.

In the embodiments of the invention shown in FIGS. 2 and 3 only onemixer and one amplifier are necessary to mix and amplify return echosignals from either a target or chaff. FIG. 2 illustrates a systemwherein one 3 transmitting antenna 3] is horizontally polarized totransmit the carrier signal from the oscillator 32 that is frequencymodulated by either a band of white noise or a periodic wave from themodulator 33.

The receiving antenna array comprises at least one horizontallypolarized antenna 34 and one vertically polarized antenna 35. The returnecho signal received by the antennas 34 and 35 are passed on to themixer 36 through a ferrite switch 37 which alternately connects theantennas 34 and 35 with the mixer 36. The return echo signal passed tothe mixer is mixed with a portion of the transmitted signal and fed tothe band pass amplifier 39 which is set to pass a band of expectedDoppler frequencies to either of the envelope detectors 44 or 42depending upon the position of the switch 43. The switch 43 and theferrite switch 37 are actuated by a square wave generator 44 whereby theoutput from the amplifier 39 is passed to the horizontal envelopedetector 41 whenever the horizontal polarized receiving antenna 32 isconnected to the mixer 36 and likewise the output from the amplifier 39is passed to the vertical envelope detector 42 whenever the verticallypolarized antenna 35 is connected to the mixer 36. The output from thehorizontal and vertical envelope detectors 4] and 42 are applied to adifference amplifier 45 to produce a signal from the differenceamplifier 45 to the firing circuit 46 only when a target is encountered.

in operation of the embodiment shown in FIG. 2 the transmitted signalfrom the antenna 31 is horizontally polarized whereby a signal reflectedfrom a target surface will have only a horizontal polarized componentwhile a reflected signal from chaff will have both a horizontal andvertical component. The receiving antennas 34 and 35 are alternatelyconnected to the mixer 36 whereby a reflected signal from a targetsurface would result in an output from the horizontal envelope detector41 and none from the vertical polarized signal. The difference amplifier45 is designed to produce a firing signal for the firing circuit 46whenever the amplitude of the horizontal envelope is greater than thevertical envelope detected from the amplifier 39 output whereby a firingsignal occurs when the return target echo signal is received since thereis no vertical component to cancel out the horizontal component.

When a return echo is received from chaff it will comprise bothhorizontal and vertical polarized components and the energy output fromthe envelope detectors 41 and 42 will be equal in amplitude and will notproduce a firing signal from the difference amplifier 45. in order toinsure that no firing signal is produced by the difference amplifier 45the gain of the detector 42 is normally greater than the detector 41whereby the energy level of the vertical envelope will always be equalto or greater than the horizontal enve lope.

The embodiment shown in FIG. 3 is the same as the embodiment shown inFIG. 2 except ferrite switch 51 is provided to alternately switch theoutput from the oscillator 32 to a horizontally polarized transmittingantenna 52 and a vertically polarized transmitting antenna 53. Also ahorizontally polarized receiving antenna 54 receives the return echosignal and applies it to the mixer 36, and square wave output from theoscillator 44 is applied to synchronously switch the ferrite switch 51and switch 43.

In operation of the embodiment shown in FIG. 3 the switches 43 and 51are synchronized so that the amplifier 39 output is connected to thehorizontal envelope detector 41 at the same time that the horizontallypolarized signal is transmitted from the antenna 52 and the output fromamplifier 39 is applied to the vertical envelope detector 42 when avertically polarized signal is transmitted from the antenna 53. Therelationship of the detectors 41, 42 and the difference amplifier 45 isthe same as in the embodiment of FIG. 2 wherein a fir ing signal isproduced by the differential amplifier 45 whenever the energy level ofthe envelope detected by the horizontal detector is greater than theenergy level of the detector 42 output. Therefore, when a horizontallypolarized signal is transmitted by antenna 52 a horizontally polarizedsignal will be received from a target aircraft and the switch 43 willconnect the output from the amplifier 39 to the horizontal envelopedetector. The vertically polarized signal from the antenna 53, reflectedfrom the target and received by the antenna 54 will have a very smallamount of energy, if any, passed by the antenna 54 to the circuit 36-42.Since there will be substantially no output from the detector 42 appliedto the difference amplifier 45, a firing signal will be produced by theoutput of the detector 41 applied to the difference amplifier 45.

When chaff reflects the horizontal and vertical polarized transmittedsignals there will be both a reflected horizontal and vertical componentof equal magnitude and by raising the gain of the detector 42 above thegain of the detector 41 the output from detector 42 will prevent theoutput detector 41 from producing a firing signal when the outputs areapplied to the difference amplifier 45.

Although the above embodiments shown in FIGS. 1-3 are shown with fuzesystems having a modulated transmitted signal it is to be understoodthat the invention can be used with unmodulated transmitted signalswithout varying from the scope of the invention. Also the systems showncan be used as a counter-countermeasure device where the enemy used acircularly polarized signal for jamming a fuze or guidance system.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

l. A microwave fuze system for a missile including a firing circuithaving means for discriminating between a target and a countermeasurereflector, comprising in combination, a frequency modulated transmitter,an antenna coupled to said transmitter and radiating a signal polarizedin a predetermined direction, a pair of receiving antennas including oneantenna polarized in the same direction as said transmitting antenna andone antenna polarized in a direction normal to said transmittingantenna, a first switch means coupled to said receiving antennas andadapted to alternately connected said receiving antennas to a mixingmeans whereby the received signals are mixed with a portion of thetransmitted signal, means for amplifying a band of said mixedtransmitted and received signals, a pair of envelope detector meansconnected to said amplifying means by a second switch means, adifference amplifier adapted to receive the output from said pair ofenvelope detector for coupling a firing signal to said firing circuitwhen the energy level of the output of a certain one of said detectorsexceeds the energy level of the other detector output, and a periodicwave control means actuating said first and second switch means wherebysaid pair of receiving antennas and said pair of envelope detectors aresynchronously connected to

1. A microwave fuze system for a missile including a firing circuithaving means for discriminating between a target and a countermeasurereflector, comprising in combination, a frequency modulated transmitter,an antenna coupled to said transmitter and radiating a signal polarizedin a predetermined direction, a pair of receiving antennas including oneantenna polarized in the same direction as said transmitting antenna andone antenna polarized in a direction normal to said transmittingantenna, a first switch means coupled to said receiving antennas andadapted to alternately connected said receiving antennas to a mixingmeans whereby the received signals are mixed with a portion of thetransmitted signal, means for amplifying a band of said mixedtransmitted and received signals, a pair of envelope detector meansconnected to said amplifying means by a second switch means, adifference amplifier adapted to receive the output from said pair ofenvelope detector for coupling a firing signal to said firing circuitwhen the energy level of the output of a certain one of said detectorsexceeds the energy level of the other detector output, and a periOdicwave control means actuating said first and second switch means wherebysaidpair of receiving antennas and said pair of envelope detectors aresynchronously connected to said mixing means and said amplifying meansrespectively.